From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2A462C25B47 for ; Fri, 27 Oct 2023 21:50:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232154AbjJ0Vut (ORCPT ); Fri, 27 Oct 2023 17:50:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48770 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235116AbjJ0Vuq (ORCPT ); Fri, 27 Oct 2023 17:50:46 -0400 Received: from mgamail.intel.com (mgamail.intel.com [192.55.52.115]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 646911BF; Fri, 27 Oct 2023 14:50:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1698443444; x=1729979444; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; 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27 Oct 2023 14:48:12 -0700 Message-ID: <9fe968f0-cf38-4abf-b01c-5591c97ec886@intel.com> Date: Fri, 27 Oct 2023 14:50:41 -0700 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 05/50] x86/speculation: Do not enable Automatic IBRS if SEV SNP is enabled Content-Language: en-US To: Michael Roth , kvm@vger.kernel.org Cc: linux-coco@lists.linux.dev, linux-mm@kvack.org, linux-crypto@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com, jroedel@suse.de, thomas.lendacky@amd.com, hpa@zytor.com, ardb@kernel.org, pbonzini@redhat.com, seanjc@google.com, vkuznets@redhat.com, jmattson@google.com, luto@kernel.org, dave.hansen@linux.intel.com, slp@redhat.com, pgonda@google.com, peterz@infradead.org, srinivas.pandruvada@linux.intel.com, rientjes@google.com, dovmurik@linux.ibm.com, tobin@ibm.com, bp@alien8.de, vbabka@suse.cz, kirill@shutemov.name, ak@linux.intel.com, tony.luck@intel.com, marcorr@google.com, sathyanarayanan.kuppuswamy@linux.intel.com, alpergun@google.com, jarkko@kernel.org, ashish.kalra@amd.com, nikunj.dadhania@amd.com, pankaj.gupta@amd.com, liam.merwick@oracle.com, zhi.a.wang@intel.com, Kim Phillips References: <20231016132819.1002933-1-michael.roth@amd.com> <20231016132819.1002933-6-michael.roth@amd.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/16/23 06:27, Michael Roth wrote: > Without SEV-SNP, Automatic IBRS protects only the kernel. But when > SEV-SNP is enabled, the Automatic IBRS protection umbrella widens to all > host-side code, including userspace. This protection comes at a cost: > reduced userspace indirect branch performance. > > To avoid this performance loss, don't use Automatic IBRS on SEV-SNP > hosts. Fall back to retpolines instead. Thanks for the updated changelog: Acked-by: Dave Hansen BTW, have you given your hardware folks a hard time about this? It seems _kinda_ silly to be using retpolines when the hardware has a perfectly good IBRS implementation for the kernel. Just please make sure there's a good underlying reason for this behavior and as opposed to being some kind of inadvertent side effect. I assume Auto-IBRS and SEV-SNP are going to be with us for a long time, so it would be nice to have a long term solution here.